Zero-voltage complementary switching high efficiency class D amplifier

ABSTRACT

A high-efficiency Class D amplifier is described. In one embodiment, the amplifier contains two transistors which are turned on and off in sequence, the output being taken from the common node between the transistors. The efficiency of the amplifier is substantially increased by insuring that voltage across each of the transistors is substantially zero when it turns on. This advantage is achieved by connecting an inductance to the common node, so that the electrical energy stored in any stray capacitance is transferred to the inductance before any transistor is turned on.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to, and incorporates by reference, thefollowing U.S. patent applications filed on the same date as the presentapplication: the application entitled "Electrodeless Discharge Lamp withSpectral Reflector and High Pass Filter" filed by Nicholas G. Vrionis,Ser. No. 07/887,165 filed May 20, 1992, the application entitled "RadioFrequency Interference Reduction Arrangements for ElectrodelessDischarge Lamps" filed by Nicholas G. Vrionis and Roger Siao, Ser. No.07/883,850 filed May 20, 1992; the application entitled "PhosphorProtection Device for Electrodeless Discharge Lamp" filed by Nicholas G.Vrionis and John F. Waymouth, Ser. No. 07/883,972 filed May 20, 1992;the application entitled "Base Mechanism to Attach an ElectrodelessDischarge Light Bulb to a Socket in a Standard Lamp Harp Structure"filed by James W. Pfeiffer and Kenneth L. Blanchard, Ser. No. 07/884,000filed May 20, 1992, the application entitled "Fluorescent Light Bulbsand Lamps and Methods for Making fluorescent Light Bulbs and Lamps",filed by Nicholas G. Vrionis, Ser. No. 07/883,971 filed May 20, 1992;the application entitled "Stable Power Supply in an ElectricallyIsolated System Providing a High Power Factor and Low HarmonicDistortion" filed by Roger Siao, Ser. No. 07/886,718 filed May 20, 1992;and the application entitled "Impedance Matching and Filter Network forUse With Electrodeless Discharge Lamp" filed by Roger Siao, Ser. No.07/887,166 filed May 20, 1992.

FIELD OF THE INVENTION

This invention relates to Class D amplifiers and in particular tohigh-efficiency Class D amplifiers which are suitable for providing ahigh-frequency signal to an induction coil in an electrodeless dischargelamp.

BACKGROUND OF THE INVENTION

A basic form of Class D amplifier is illustrated in FIG. 1A. Twotransistors Q_(A) and Q_(B) are driven by a transformer to switch on andoff 180 degrees out of phase with each other. The two transistors incombination are equivalent to the double-pole switch illustrated in FIG.1B, and at their common node produce a square-wave output similar to thewaveform illustrated in FIG. 1C.

Ideally, a Class D amplifier should be 100% efficient, i.e., no powershould be consumed in transistors Q_(A) or Q_(B) as they are repeatedlyswitched on and off. In reality, however, transistors Q_(A) and Q_(B)have their own on-stage resistances, commonly known as the on-resistanceof the transistor. They also have inherent capacitances, illustrated ascapacitors C_(A) and C_(B) in FIG. 1A, which permit charge to build upwhen either transistor is turned off. Therefore, a voltage differenceexists across the transistor when it is turned on, and the resultingcurrent flow through the transistor dissipates energy as heat. Thisenergy loss can be expressed as cv² f, where c is the inherentcapacitance of the transistor (the value of C_(A) or C_(B)), v is the DCinput voltage (V_(DD)) and f is the frequency at which the amplifier isdriven. In reality, high frequency Class D amplifiers typically operateat an efficiency of about 50% to 60%. A large portion of this efficiencyloss is attributable to the inherent capacitances of the transistors.This inefficiency has seriously limited the suitability of Class Damplifiers for electrodeless discharge lamps and other devices in whichsignificant power losses cannot be tolerated.

In actual operation, transistors Q_(A) and Q_(B) having the sameelectrical characteristic, do not switch off and on instantaneously andsimultaneously as implied in FIG. 1C. Rather, the input from thetransformer is typically sinusoidal and turns each transistor on when itreaches its threshold voltage V_(th), as indicated in FIG. 1D.Accordingly, there is a lag between the time when one of the transistorsturns off and the other transistor turns on. The output signal istherefore not a perfect square wave but instead has sloped transitionsas illustrated in FIG. 1E, the time lag being denoted as Δt. Moreover, afinite time is required to turn each transistor on or off.

SUMMARY OF THE INVENTION

In a Class D amplifier in accordance with this invention, an inductance,internal to the amplifier, is capacitively coupled to the common nodebetween the two transistors. The inductance in effect forms a resonantcircuit with the inherent capacitances of the transistors as well asother stray capacitances present in the circuit, all of which arereferred to together as the output capacitance C_(O). The value of theinductance is selected such that, in the interval (Δt) during which bothtransistors are turned off, the power stored in the output capacitanceC_(O) is transferred to the inductance. Thus, when one transistor isturned off, an energy transfer takes place between the outputcapacitance C_(O) and the inductance such that when the other transistorturns on there is no voltage drop across it. As noted above, this is thecondition required to minimize power losses due to the switching of thetransistors.

The resonant circuit formed with the inductance and the outputcapacitance thus causes the energy stored in the output capacitance tobe transferred to the inductance rather than being dissipated by acurrent flow and heat loss when the transistor is turned on.

The principles of this invention are particularly applicable to devicessuch as electrodeless discharge lamps, where it is important to theoverall efficiency of the lamp to minimize the power lost in amplifyingthe oscillating signal that is delivered to the induction coil.

DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a circuit diagram of a conventional Class Damplifier.

FIG. 1B illustrates an equivalent circuit for the Class D amplifier.

FIG. 1C illustrates an idealized output of the Class D output amplifier.

FIG. 1D illustrates the input from the signal source to the Class Damplifier.

FIG. 1E illustrates the output of the Class D amplifier taking intoaccount switching delays.

FIG. 2 illustrates a circuit diagram of a Class D amplifier inaccordance with the invention.

FIG. 3 illustrates the transformer in the Class D amplifier of FIG. 2.

FIG. 4A is a superimposed view of the input and output signals of aClass D amplifier.

FIG. 4B illustrates the resonant circuit in the Class D amplifier ofFIG. 2.

FIG. 5 illustrates a block diagram of a Class D amplifier in accordancewith this invention connected to an electrodeless discharge lamp.

DESCRIPTION OF THE INVENTION

FIG. 2 illustrates a circuit diagram of a high-efficiency Class Damplifier in accordance with this invention. A signal source S deliversa sinusoidal input signal, R_(S) representing the impedance of signalsource S. This input signal is delivered to a transformer assembly T₁which includes transformers T_(1a) and T_(1b) sharing the same core CR,as shown in FIG. 3. The windings of transformers T_(1a) and T_(1b) areidentified as L_(a), L_(b), L_(c) and L_(d) in FIGS. 2 and 3. (L_(a)-L_(d) sometimes refer to the self-inductance of each individualwinding.) As illustrated in FIG. 3, windings L_(a) and L_(b) arebifilarly wound around one side of a core CR, and windings L_(c) andL_(d) are bifilarly wound around the other side of core CR. The pair ofwindings L_(a) and L_(b) are wound around core CR in the same directionas the windings L_(c) and L_(d). The coupling between windings L_(a) andL_(b) (transformer T_(1a)) and between windings L_(c) and L_(d)(transformer T_(1b)) is nearly unity, while because transformers T_(1a)and T_(1b) are positioned on opposite sides (approximately 180 degreesapart) of core CR, and because the permeability of core CR is very low,the coupling between them is very low (typically about 0.15). Forreasons described below, Transformer T_(1a) has more turns thantransformer T_(1b).

The switching function is provided by transistors Q₁ and Q₂, each ofwhich is an N-channel power MOSFET. Transistors Q₁ and Q₂ are connectedin series between a DC supply voltage V_(DD) and ground, with the sourceterminal of transistor Q₁ and the drain terminal of transistor Q₂forming a common node which represents the output of the amplifier. Theoutput is fed to an impedance matching network N and a load L. Impedancematching network N may be of several varieties known to those skilled inthe art and is not a part of this invention.

Winding L_(a) has an end E which is AC coupled to the gate of transistorQ₁ through a coupling capacitor C₁. An end F of winding L_(b) is ACcoupled to the output through a coupling capacitor C₂, and an end D ofwinding L_(d) is connected to the gate terminal of transistor Q₂ throughan inductance L₁. Capacitors C₁, C₂ and C₃ shown in FIG. 2 serve as ACcoupling capacitors. Capacitor C₄ serves as an AC bypass capacitor whichensures that the AC impedance between the drain terminal of transistorQ₁ and current ground is maintained at a minimum. Resistors R₁ and R₂are connected between the gate and source terminals of transistors Q₁and Q₂, respectively, and ensure that the gates of those transistors aremaintained at a DC zero bias.

Transformer T_(1a) forms a Balun transmission-line transformer, whichinverts the signal from source S and applies the inverted signal acrossthe gate of transistor Q₁. (Balun transformers are described in SolidState Radio Engineering, by Herbert L. Krauss, et al., John Wiley &Sons, 1980, p. 374, which is incorporated herein by reference.) On theother hand, transformer T_(1b) is a conventional transformer, whichdelivers a signal to the gate of transistor Q₂ that is in phase with thesignal from source S. Thus, when the signal from source S goes high theoutput of transformer T_(1b) at end D also goes high and turnstransistor Q₂ on. At the same time, the output from transformer T_(1a)at end E referenced to end F goes low turning transistor Q₁ off. Thearrangement of the Balun transformer T_(1a) and the regular transformerT_(1b) on core CR in the manner shown helps to ensure that there isadequate separation between the time when one of transistors Q₁ and Q₂turns off and the other transistor turns on. In addition, inductance L₁imposes a phase delay in the signal applied to the gate of transistor Q₂and further insures that transistors Q₁ and Q₂ will both be turned offfor some period of time at each transition of the output signal (or tominimize overlapping between transistors Q₁ and Q₂).

Capacitor C_(O), shown in hatched lines, represents the total outputcapacitance of the amplifier. Thus it includes both the inherentcapacitances of transistors Q₁ and Q₂ (comparable to capacitors C_(A)and C_(B) in FIG. 1A) as well as any other stray capacitances in thecircuit.

FIG. 4A illustrates a graph of the input signal from source Ssuperimposed upon the output signal, and FIG. 4B illustrates inidealized form a resonant circuit found within the Class D amplifier.FIG. 4A shows in particular the time interval Δt during which the inputsignal is less than the threshold voltage V_(th) of either oftransistors Q₁ and Q₂. As noted above, this represents the time periodin which both transistors are turned off. While this is the case, thecommon node between transistors Q₁ and Q₂ floats, and a resonant circuitis in effect established. As illustrated in FIG. 4B, this resonantcircuit includes capacitor C_(O), inductor L_(b) (which is a part oftransformer T_(1a)) and the equivalent series resistance R_(e) of thecharge-circulating path formed by L_(b) and C_(O). For small values ofR_(e), the natural frequency f_(n) of this circuit is governed by thefollowing relationship: ##EQU1##

Thus, at the instant both transistors Q₁ and Q₂ turn off the chargebuilt up on capacitor C_(O) begins to discharge through the resonantcircuit illustrated in FIG. 4B, at a rate determined by the naturalfrequency of the circuit. The desired condition is that the voltageacross each of transistors Q₁ and Q₂ will be equal to zero when thetransistors turn on. The time it takes for the voltage across C_(O) tofall from +V_(DD) to zero is approximately equal to one-fourth of thelength of a single cycle at the natural frequency F_(n). Thus, if thevoltage across C_(O) is to be zero when transistor Q₂ turns on, thefollowing relationship should obtain

    Δt=1/4f.sub.n                                        (2)

Combining equations (1) and (2) gives the required value of L_(b).##EQU2##

If this relationship is maintained, in the steady state the energystored in C_(O) will be transferred to the inductance L_(b) during theperiod Δt when both transistors are turned off, rather than beingdissipated as heat generated by a current flow through one oftransistors Q₁ or Q₂ when it is turned on. The stored energy istransferred back and forth between C_(O) and L_(b) (the "flywheel"effect) instead of being dissipated.

In the preferred embodiment, the following relationship should obtain

    L.sub.a =L.sub.b >>L.sub.c =L.sub.d

Ideally, the values of L_(a) or L_(b) are approximately ten times thevalues of L_(c) or L_(d). This ensures that the energy reflected backfrom the output of the amplifier to the common input node (A and C) issubstantially attenuated. Windings L_(b) and L_(c) are equivalent to avoltage divider between the output terminal and circuit ground. As aresult, the input driving power supplied by source S is minimized whilethe amplifier exhibits better stability. Also, giving winding L_(c) arelatively low value as compared with winding L_(b) allows the value ofL_(c), as well as the low impedance of source S, to be ignored incalculating the resonant frequency of the circuit, as illustrated inFIG. 4B. Further, the low value of L_(c) ensures that the impedance atthe common node is relatively fixed by the low impedance of windingL_(c). Windings L_(c) and L_(d) should, however, provide sufficientimpedance for source 10.

The Class D amplifier of this invention is useful with any type ofdevice which requires high-efficiency amplification. It is particularlyuseful with an electrodeless discharge lamp, as illustrated in FIG. 5.As described in U.S. Pat. No. 4,010,400 to Hollister, incorporatedherein by reference, electrodeless discharge lamps typically include aninduction coil which is energized at a high frequency so as to transferenergy to a gaseous mixture by means of an electromagnetic field. Thegaseous mixture, which typically includes mercury vapor and an inertgas, is contained within a sealed vessel, the inside surfaces of whichare coated with phosphors. When so energized, the atoms of mercury vaporare excited and emit radiation (primarily in the UV spectrum), which inturn causes the phosphors to emit visible light.

As shown in FIG. 5, a power supply 50 supplies power to an oscillator 51and an amplifier 52 in accordance with this invention. The oscillatornormally operates at 13.56 MHz, which is a frequency set aside by theFCC for industrial, scientific and medical (ISM) applications. Theoutput of amplifier 52 is passed through a filter and matching network53 to an induction coil network 54, which is positioned within a centralcavity of a glass vessel 55. Network 53 is preferably the impedancematching and filter network described in application Ser. No. 07/887,166filed May 20, 1992.

To illustrate the benefits of this invention as applied to electrodelessdischarge lamps, assuming a DC supply voltage of 130 V and a frequencyof 13.56 MHz, the power losses according to the formula cv² f wouldtypically equal about 9 watts for a total capacitance C_(O) =40 pf. Thisis about half the rated power consumption of a 19 watt bulb. Using thetechniques of this invention, the efficiency of the amplifier can beincreased from 50% or 60% to about 95%, and the power loss falls toabout 1 watt.

The embodiment described above is intended to be illustrative and notlimiting. Numerous alternative embodiments will be apparent to thoseskilled in the art, and all such alternative embodiments are intended tobe within the broad scope of this invention, as defined in the followingclaims. For example, although the electrodeless discharge lamp describedin U.S. Pat. No. 4,010,400 has been referred to, the amplifier of thisinvention may be used with other types of electrodeless discharge lamps.Moreover, the principles of this invention are applicable toelectrodeless discharge lamps in which the visible light is generateddirectly from the enclosed gas rather than by a coating of phosphorsapplied to the surface of the enclosing vessel.

I claim:
 1. An amplifier comprising:first and second switching means connected in series; a switch control means for controlling said first and second switching means; and a signal source connected to said switch control means, said switch control means operative to cause each of said first switching means and said second switching means to open and close in sequence, one of said first and second switching means being open whenever the other of said first and second switching means is closed, and there being a time interval during which both of said first and second switching means are open; wherein said amplifier has an inherent capacitance which stores energy when one of said first and second switching means is open; said switch control means comprising a means for storing energy, said means for storing energy being operative during said time interval to receive energy stored in said inherent capacitance so as to reduce the voltage across each of said first and second switching means to approximately zero when it changes from an open to a closed condition.
 2. The amplifier of claim 1 wherein said switch control means comprises a transformer having a primary winding and a secondary winding, said means for storing energy comprising said primary winding.
 3. The amplifier of claim 2 wherein said first switching means comprises a first transistor and said second switching means comprises a second transistor.
 4. The amplifier of claim 2 wherein said transformer comprises a Balun transformer.
 5. The amplifier of claim 4 wherein said switch control means further comprises a conventional transformer, said Balun transformer being connected to said first switching means and said conventional transformer being connected to said second switching means.
 6. The amplifier of claim 5 wherein said conventional transformer and said Balun transformer each comprise a primary winding and a secondary winding, said primary and secondary windings of both said conventional transformer and said Balun transformer being wound on a toroidal core.
 7. The amplifier of claim 6 wherein the primary winding of said Balun transformer has a self inductance L_(b) which is substantially equal to a self inductance L_(a) of the secondary winding of said Balun transformer, the primary winding of said conventional transformer has a self inductance L_(c) which is substantially equal to a self inductance L_(d) of the secondary winding of said conventional transformer, and each of L_(a) and L_(b) is substantially greater than each of L_(c) and L_(d).
 8. The amplifier of claim 7 wherein the ratio L_(c) /L_(b) constitutes an attenuation factor that represents a reduction in a positive feedback signal that is delivered from a common node between said first and second switching means to a common node between the respective primary windings of said Balun and conventional transformers.
 9. The amplifier of claim 7 comprising an inductor connected between said conventional transformer and said second switching means.
 10. The amplifier of claim 9 wherein said inductor has a value selected so as to impose a phase delay in the signal applied to said second gating means.
 11. The amplifier of claim 1 wherein said means for storing energy comprises an inductance having a value substantially equal to L_(b) such that ##EQU3## wherein Δt is the length of said time interval in seconds and C_(o) is the value of said inherent capacitance of said amplifier.
 12. An electrodeless discharge lamp comprising:an amplifier according to claims 1, 2, 3, 4, 11, 5, 6, 7, 8 or 9; and an induction coil positioned adjacent to a sealed vessel, said sealed vessel containing gas comprising a metal vapor, said amplifier being connected to said induction coil.
 13. The electrodeless discharge lamp of claim 12 wherein said induction coil is positioned within a central cavity formed by an exterior surface of said vessel.
 14. The electrodeless discharge lamp of claim 12 wherein said sealed vessel is coated with phosphors.
 15. The electrodeless discharge lamp of claim 12 wherein visible light is generated by the gas contained in said sealed vessel.
 16. The amplifier according to claim 1 wherein the means for storing energy comprises an inductance having a value selected such that substantially all the energy stored in said inherent capacitance is transferred to said inductance during said time interval.
 17. An electrodeless discharge lamp comprising:a source of an oscillating signal; an induction coil positioned adjacent to a sealed vessel, said sealed vessel containing a metal vapor; and an amplifier connected to said induction coil, said amplifier comprising:first and second switching means connected in series; and a switch control means for sequentially opening and closing said first and second switching means; wherein said amplifier has an inherent capacitance which stores energy when one of said first and second switching means is open; said switch control means comprising a means for storing energy, said means for storing energy being operative to receive energy stored in said inherent capacitance so as to reduce the voltage across each of said first and second switching means to substantially zero when each of said first and second switching means changes from an open condition to a closed condition.
 18. The electrodeless discharge lamp of claim 17 wherein said induction coil is positioned within a central cavity formed by an exterior surface of said vessel.
 19. The electrodeless discharge lamp of claim 18 wherein said means for storing energy comprises an inductance.
 20. The electrodeless discharge lamp of claim 19 wherein said first switching means comprises a first transistor and said second switching means comprises a second transistor.
 21. The electrodeless discharge lamp of claim 20 wherein said switch control means comprises a Balun transformer, said means for storing energy being at least partially included within said Balun transformer.
 22. The electrodeless discharge lamp of claim 21 further comprising a conventional transformer wherein said Balun transformer and said conventional transformer are arranged so as to ensure that said first and second switching means are open during a time interval. 